1. Field of the Invention
The present invention relates to a polyimide used for melt-processing, and more particularly relates to a polyimide having excellent processability, and to a process for preparing the same.
2. Description of the Related Art
Polyimide obtained by reacting a tetracarboxylic dianhydride with a diamine has very high heat-resistance and is additionally excellent in mechanical strength, dimensional stability, flame retardancy and electrical insulation. On account of such favorable performance, polyimide has been conventionally used in the field of electric and electronic apparatus, space and aeronautic equipment and transportation machinery. Thus, polyimide is a functional resin which is expected to be widely used in fields where heat resistance is required. New fields of use have required diversified performance and application methods. Various kinds of polyimide having excellent characteristics have been developed to meet these demands.
Some of the polyimides, however, have no definite glass transition temperature, though excellent in heat resistance. Consequently, the polyimide must be processed by such means as sinter molding, when it is used as a molding material. Other kinds of polyimides are soluble in solvents such as halogenated hydrocarbons, and though excellent in processability, have a disadvantage in solvent resistance. Thus, conventionally developed polyimides have both merits and drawbacks in its performance.
Conventionally known polyimides have been commonly difficult to process. These kinds of polyimide are thermosetting resins. Hence it is difficult to apply melt-processing methods which can operate with ease and a specialized processing method such as sintering must be employed.
For example, polyimide prepared from pyromellitic dianhydride and 4,4'-diaminodiphenyl ether and having a fundamental skeleton of the following formula (I): ##STR2## has no distinct glass transition temperature and is difficult to process as a molding material, though excellent in heat resistance. It has been well known that this polyimide must be processed by means of sintering.
Another polyimide obtained by reacting pyromellitic dianhydride with 1,4-bis(4-aminophenoxy)benzene has been known for a long time as disclosed in SU-188,005. The polyimide, however, exhibits no melt-flowability at all at 450.degree. C. and is quite difficult to process in the molten state.
Further, polyimide prepared from 3,3',4,4'-diphenylethertetracarboxylic dianhydride and 1,4-bis(4-aminophenoxy)benzene has been disclosed in U.S. Pat. No. 4,855,391 and EP 283,853. However, no description has been found at all on the melt-flowability and injection molding of the polyimide.
Therefore, in order to extend utilization of polyimides in fields where the excellent properties of polyimide can be applied, particularly in the high technology field, it is extremely important to discover polyimide having a specific structure which can be melt processed while maintaining the various excellent characteristics of polyimide.